The 3GPP Release 5 IP Multimedia Domain

Figure 6-29 shows the network architecture for a new core network domain
planned for 3GPP Release 5. This architecture has already been described
in Chapter 4, “Third Generation (3G) Overview.” It is important to note that
this architecture represents an addition to the core network, rather than a
change to the existing core. Instead, 3GPP Release 5 introduces a new core
network domain in addition to the established CS and PS domains. This
new domain is available for new user devices that have the capability and
the call model logic needed to take advantage of the new domain. Thus, the
UTRAN can now be connected to three different logical core network
domains—the CS domain, the PS domain, and the IP Multimedia (IM)
domain. When a terminal wants to use the services of the core network, it
indicates which domain it wants to use. Existing (pre-Release 5) terminals
will continue to request the services of the CS or PS domain. New terminals
will also be able to request the services of the IM domain.
Note that while the IM domain is a new domain, it uses the services of
the PS domain. All IM traffic is packet based and is transported using PSdomain
nodes such as the SGSN and GGSN.
The IM domain is based on the Session Initiation Protocol (SIP), as
described in Chapter 8. In fact, the Call State Control Function (CSCF) of
Figure 6-29 is effectively a SIP proxy. The IM architecture enables voice and
data calls to be handled in a uniform manner all the way from the UE to the
destination. A complete convergence of voice and data takes place, such that
voice is simply a type of data with specific QoS requirements. This convergence
enables a number of new advanced services. Moreover, the use of SIP
means that a great deal of service control can be placed in the UE rather
than the network, making it easier for the subscriber to customize services
to meet his or her particular needs.